1. Field of the Invention
The invention relates to methods and structures utilized in the fabrication of high quality Group III-nitride materials. Embodiments of the invention are related to methods for improving the surface quality of Group III-nitride semiconductor material. In addition the invention also encompasses structures fabricated utilizing the methods of the invention.
2. Background of the Invention
The quality of a semiconductor material can considerably impact the performance of a solid state device produced from said material. Solid state devices can suffer from inferior lifetimes and operating characteristics when the semiconductor material encompasses an undesirable density of crystal defects, for example dislocations. Monocrystalline semiconductor wafers for device fabrication are commonly sliced from bulk single crystals. However, for certain materials, bulk single crystals are difficult or impossible to grow, and monocrystalline wafers of such materials are often grown epitaxially on suitable substrates. A suitable substrate is one that closely matches the crystal properties of the target material to be grown; if these properties do not closely match, the resulting material usually has an unacceptable density of defects and dislocations.
It has turned out in practice that, for semiconductor materials such as gallium nitride (GaN), other Group III-nitrides (e.g., AlN, InN, GaInN) and other mixed nitrides (referred to herein as “Group III-nitrides”), neither bulk single crystals nor suitable substrates are available. In the case of GaN, crystal quality can be improved to some extent by pre-treatment of the growth substrates, e.g., by nitridization and other chemical modifications; by growing thin, low temperature buffer layers of other Group III-nitrides, e.g., AlN or GaN, by thermal annealing, and the like.
Also GaN growth methods such as epitaxial lateral overgrowth (ELO) and its variants (PENDEO, FIELO, etc) have proven successful in reducing defect density. However these methods often produce materials with a highly non-uniform distribution of surface defects, undesirable in many device applications. Newer growth methods of limiting defect number and improving their surface homogeneity have utilized in-situ (or ex-situ) deposition methods to impede dislocation progression in some instances along with the optional addition of etchants to enhance surface defect dimensions. See, e.g. Applied Materials US2007/0259504, Tanaka et al. Japanese Journal of Applied Physics 39 L831 2000 and Zang et al. Journal of Applied Physics 101 093502 2007).
Accordingly, in the past wafers of Group III-materials have had limited availability along with insufficient quality to serve either as substrates for fabrication of reliable devices or as substrates for growth of further Group III-nitride materials. Clearly, layers and crystals of Group III-nitrides of improved quality are desirable, but widely applicable processes for doing so are not commonly known in the prior art.